Fluid catalytic cracking processes (or “FCC” processes as used herein) are known in the art and such processes are used primarily for petroleum and petrochemical conversion processes. These processes are valued due to their ability for efficient and selective catalytic cracking of hydrocarbon containing feedstock by fluidizing small catalyst particles and mixing with a feedstock by intimate contact under thermally active conditions to generally produce lower molecular weight “cracked” products. FCC processes are beneficial due to their ability to continuously recycle and regenerate the spent catalysts and to process large volumes of hydrocarbon containing feedstock.
Conversion of high molecular weight petroleum feeds to more valuable products by catalytic processes such as fluidized catalytic cracking are important processes to the petroleum and petrochemical industries. In these fluidized catalytic cracking process, higher molecular weight feeds are contacted with fluidized catalyst particles most advantageously in the riser reactor of the fluidized catalytic cracking unit. The contacting between feed and catalyst is controlled according to the type of product desired. In catalytic cracking of the feed, reactor conditions such as temperature and catalyst circulation rate are controlled to maximize the products desired and minimize the formation of less desirable products such as light gases and coke.
Miscellaneous fluidized catalytic cracking reactor riser and reactor vessel designs have been utilized in the past. However, with the advance of zeolitic cracking catalysts with greatly improved cracking activity, most modern fluidized catalytic cracking reactors utilize a short contact-time cracking configuration. With this configuration, the time in which the catalyst and the fluidized catalytic cracker feedstream are in contact is limited in order to minimize the amount of excessive cracking which results in the increased production of less valued products such as light hydrocarbon gases as well as increased coking deposition on the cracking catalysts.
Most fluidized catalytic cracking configurations utilize a reactor riser cracking configuration wherein the catalyst is contacted with the fluidized catalytic cracker feedstock in a reactor riser, and the catalyst and the hydrocarbon reaction products are separated shortly after the catalyst and hydrocarbon mixture leaves the reactor riser and enters the fluidized catalytic cracking reactor. Although there are many different fluidized catalytic cracking reactor designs in use, most use mechanical cyclones internal to the reactor to separate the catalyst from the hydrocarbon reactor products as quickly and efficiently as possible. This rapid separation process has the benefits of both minimizing post-riser reactions between the catalyst and the hydrocarbons as well as providing a physical means for separating the cracked hydrocarbon products for further processing from the spent catalyst which is regenerated prior to reintroduction of the regenerated catalyst back into the reaction process.
The catalyst that is separated from the cracked hydrocarbon products in the FCC reactor is considered as “spent catalyst” until such time as the catalyst can typically be sent to an FCC regenerator vessel and regenerated into a “regenerated” catalyst. Prior to being regenerated, the spent catalyst is typically stripped of most of the hydrocarbon layer which remains on the catalyst after it is separated from the bulk of the FCC products. This “stripped” catalyst is then sent via a spent catalyst riser to an FCC regenerator, wherein the catalyst is subjected to an oxidizing atmosphere to burn the remaining hydrocarbons and “coke” from the spent catalyst in order to convert it to a regenerated catalyst. These hydrocarbons and coke interfere with the accessibility to the catalytic functioning sites in the catalyst and lower the catalyst's activity, and as such, these deposits must be significantly removed from the spent catalyst before returning the regenerated catalyst back to the catalyst cracking zone of the FCC process. The heat of reaction of the combustion of the coke also directly heats the catalyst and this energy, in the form of a hot catalyst, is used to drive the endothermic cracking reactions in the FCC reactor.
A problem that exists in the industry is that the FCC regenerator operates at very high temperatures and the total capacity of the regenerator unit is very often limited by the temperature in at the top of the FCC regenerator. As many FCC regenerators (but not all) use mechanical cyclones located in the top portion of the regenerator vessels, these temperatures are often referred to as “cyclone temperatures”, as often in the design of the FCC regenerators, the thermocouples which measure these temperatures are attached to some or all of the individual cyclones in the FCC regenerator vessel. As such, as used herein the term “cyclone temperatures” will mean the temperature in the top of the regenerator, even if the regenerator does not contain cyclones. This maximum cyclone temperature limit is often the determining limit on the maximum throughput through the regenerator vessel and as such may be a limiting factor to the total production of the FCC unit. In the FCC regenerator, most of the remaining hydrocarbons and coke on the spent catalyst is burned off due to combustion when the hot spent catalyst is exposed to the oxidizing atmosphere (most often significantly comprising air) in the regenerator vessel. The heat of combustion from this reaction causes the regenerator temperatures to rise. As the “cyclone” (or overhead) temperatures approach their maximum allowable operating temperatures, the throughput of the catalyst through the regenerator becomes “temperature limited”. These limitations on the cyclone temperatures are usually due to mechanical constraints in the metallurgy and/or refractory of the regenerator vessel and associated internal components.
Much of the excess (and unwanted) temperature in the top of the regenerator (or cyclone temperatures), is due to what is termed as “afterburn”. In the regenerator, it is desired that all of the spent catalyst and oxidizing gas (or “oxygen” herein) come in to contact and burn in the lower portion (also called the “bed” or “dense phase”) of the regenerator. In this manner, the generated heat is most effectively contained in the bottom of the regenerator where there is a large mass of catalyst in the bed to absorb the heat of combustion. Afterburn occurs when both oxygen and carbon monoxide migrate into the top (or “dilute phase”) of the regenerator. In this case, the oxygen and carbon monoxide react and the heat of combustion goes significantly into raising the cyclone temperature since in this top area of the regenerator there is not the mass of the catalyst bed to absorb the generated heat of combustion. This undesired combustion in the dilute phase (i.e., afterburn) increases the cyclone temperatures at the expense to overall throughput.
Afterburn is a significant limiting design and operation factor for FCC units. In order to prevent/minimize the amount of afterburn in the FCC units, the units are either designed for “partial burn” or “full burn” operation. In “partial burn” units, the unit is designed to operate with a deliberate, less than stoichiometric amount of oxygen in the regenerator, thus reducing the risk of oxygen in the dilute phase of the regenerator which can react with carbon monoxide and thereby reducing the amount of afterburn. In contrast, “full burn” units, the unit is designed to operate with a deliberate, more than stoichiometric amount of oxygen in the regenerator, thus reducing the risk of carbon monoxide in the dilute phase of the regenerator which can react with the excess oxygen and thereby reducing the amount of afterburn.
In either case, an operating buffer (i.e., amount excess carbon dioxide or amount excess oxygen) must be utilized to prevent significant afterburn which can cause extensive damage or failure of the regenerator components. The larger the fluctuation in the cyclone temperatures resulting from afterburn in the unit, the larger the operating buffer that must be employed to protect the equipment. However the larger the operating buffer utilized, the larger the impact there is to overall capacity on the regenerator vessel, resulting in lost throughput, capacity and, as a result, lost income.
As such, it is of significant importance that the afterburn and fluctuations in the afterburn be minimized. Minimization of these factors can result in significant financial revenues as well as significantly improve the reliability associated with an FCC unit, in particular as associated with the FCC regenerator vessel and associated equipment.
Additionally, fluid coking is a fluid cracking process family and the fluid coking regenerators are designed and operated in many ways very similar to their FCC regenerator counterparts. One main difference is that the fluid coking processes rely primarily on thermal cracking reactions between the hydrocarbon feed and the catalysts versus the primarily catalytic cracking reactions of an FCC process. This is partially due to the fact that the “catalyst” particles in the fluid coking process are primarily formed of coke particles or other mostly catalytically inert materials which primarily use the heat content in the catalyst to thermally crack the hydrocarbon feed. However, the regenerators in the fluid coking processes are very similar in design and operation as the FCC regenerators and share many of the same problems which the present invention is designed to solve.
While the afterburn problems described above for the FCC regenerators is usually not as significant a concern in the fluid coking regenerators, the same problems of undesired high temperatures in the top of the fluid coking regenerators do exist. Additionally, other problems exist in the fluid coking regenerators due to the incomplete or uneven mixing and burning of the coke from the catalyst particles such as loss of catalyst temperature used for the fluid coking reaction, as well as the excessive generation of small coke particles which get into the overhead flue gas of the fluid coking regenerator causing equipment plugging as well as environmental concerns.
Therefore, there is a need in the industry for equipment and processes to reduce the amount of afterburn and improve the combustion characteristics associated with fluid cracking regenerators, in particular FCC regenerators, and their associated process operations.